(1) Field of the Invention
The present invention relates to a method for recovering energy possessed by a blast furnace exhaust gas at a high efficiency by utilizing an axial-flow turbine, and also to an axial-flow turbine that is used for practising this method.
When a blast furnace is operated, a large quantity of an exhaust gas is generated. The temperature of this exhaust gas is considerably high and the gas has a considerable pressure. Namely, the blast furnace exhaust gas has thermal and mechanical energies. If such energies are discharged into open air together with the exhaust gas, they are wastefully lost. Thus it is desirable to recover such energies effectively.
The present invention is to provide a method and apparatus for recovering energy from a blast furnace exhaust gas by utilizing an axial-flow turbine, in which energy can be recovered at a high efficiency while preventing adhesion of dusts or clogging with dusts.
(2) Description of the Prior Art
Various attempts have heretofore been made to recover energy from a blast furnace exhaust gas. For example, one of such conventional energy recovery methods is disclosed in the specification of U.S. Pat. No. 3,818,707.
These conventional methods for recovering pressure energy from a blast furnace exhaust gas are divided into two types; one type using a centrifugal turbine and the other type using an axial-flow turbine.
One of disadvantages observed when a blast furnace exhaust gas is utilized as an operation fluid for a turbine is that since large quantities of dust are contained in the exhaust gas, if the exhaust gas is directly fed into the turbine, the dust will adhere to nozzles to cause such troubles as clogging, and the operation efficiency is reduced. Accordingly, the blast furnace exhaust gas is fed to a turbine after the dust has been removed therefrom.
In case of the method using a centrifugal turbine, an exhaust gas is first scrubbed to remove dust therefrom and the gas is saturated with water vapor and fed to the centrifugal turbine to drive it. Since in the centrifugal turbine the turbine blades are fixed more tightly than in an axial-flow turbine and nozzle blades are not provided in general, the centrifugal turbine is advantageous in that adhesion of dust is not conspicuous. However, the centrifugal turbine system is large in size and also equipment expenses are very large. Further, the operation efficiency is relatively low and it is very difficult to recover energy at a high recovery ratio.
The method using an axial-flow turbine is substantially advantageous over the method using a centrifugal turbine in the point that the size of the turbine is small and the operation efficiency is high. However, this method using an axial-flow turbine still involves various disadvantages. For example, dust adheres to nozzles (stationary blades) and moving blades to reduce the operation efficiency and cause such troubles as clogging of the nozzles. Further, when dust adheres to the moving blades, good balance cannot be maintained in the moving blades and such troubles as vibrations are caused. In some case, masses of the dust adhering to the nozzles or moving blades are separated therefrom and impinge against moving blades or other members located downstream to damage or destroy them, and there is a risk that a serious accident can take place.
In the conventional method using an axial-flow turbine, in order to prevent adhesion of dust, a blast furnace exhaust gas is passed through a wet scrubber, and the gas is heated to dry dusts and in this state the gas is introduced into the turbine. Accordingly, it is necessary to dispose a device for partial combustion of a blast furnace exhaust gas or a heat exchanger, and the equipment becomes complicated and the thermal efficiency is accordingly reduced.
The state of adhesion of dust to nozzles or other members of a turbine is varied according to such factors as the quantity of dust, the composition of the dust, the amount of water and the flow rate of the gas. Even after a blast furnace exhaust gas is treated with, for example, a wet dust precipitator (venturi scrubber), the gas contains about 100 mg/Nm.sup.3 of dust and 3 to 5 g/Nm.sup.3 of water. The dust in this state is very likely to adhere to the interior of the turbine. On the other hand, dust in the dry state is more erosive to nozzles and moving blades than dust surrounded by water and the degree of abrasion is higher. Therefore, in this case, any particular means is disposed to prevent erosion.